The invention relates to a process for the continuous preparation of filler-containing rubber powders, in which a filler suspension and rubber suspension are mixed in the aqueous phase, the mixture is passed through a tube, at least a coagulant is added in the tube and filler-containing rubber particles are precipitated.
According to the prior art, rubber mixtures are prepared from crude rubber in the form of balls with a high expenditure of time, energy and personnel. The comminution of the balls and mixing with additives are expensive and cost-intensive. On the other hand, the use of free-flowing rubber powders offers the possibility of preparing and processing rubber mixtures simply and quickly. U.S. Pat. 4,265,939 discloses a process for the continuous preparation of rubber powders, but no coagulant is mixed in a tube of relatively narrow cross-section. Effective mixing of the two components is said to take place in the narrow tube on the basis of the turbulent flow. The coagulated rubber material is then passed into a tube of widened cross-section. A polymer, as a coating composition, is fed to the liquid mixture in this tube. This known process leaves something to be desired with respect to effectively mixing the components. Furthermore, troublesome deposits or caking on the walls occur, especially in the tube section of narrowed cross-section, and these change the flow conditions. The properties of the products prepared by this process are therefore not reproducible and, because of the varying flow circumstances during the preparation these products also have disadvantageous undesirable properties. The fact that, in the context of a process for the continuous preparation of filler-containing rubber powders, the rubber material can be mixed continuously with the filler in a tube is also known in principle from DE 37 23 213 C2 and DE 37 23 214 C2. However, no fixed methodology for carrying out the process can be obtained from the two documents.
Processes for the preparation of filler-containing rubber powders which are operated batchwise or carried out discontinuously are furthermore known. In these, as a rule a filler-containing suspension is brought together with a suitable latex in the presence of a precipitant and the mixture is then allowed to react in a container. These known processes disadvantageously require a relatively high expenditure of apparatus and time. Such a discontinuous process is described, for example, in DE 28 22 148 C2.
In contrast, the invention is based on the object of providing a process for preparing a filler-containing rubber powder with reproducible properties, without problems and with a low expenditure, and to avoid the disadvantages described above.
The invention provides a process for the continuous preparation of a filler-containing rubber powder, in which
a) filler dispersions and rubber suspensions are mixed
b) the liquid mixture is passed through a tube, at least one coagulant is added and the filler-containing rubber powder is precipitated,
c) the liquid mixture is passed through a tube with at least one throttle position of variable throttle cross-section, and
d) the throttle cross-section is adjusted, with the proviso that the drop in pressure in the liquid mixture achieved with the throttle position is greater than 0.5 bar.
It lies within the context of the invention that the liquid mixture comprises at least one liquid medium, in particular water. Preferably, a suspension, expediently an aqueous suspension of the filler, is employed or added to the rubber material. A rubber latex and/or a rubber solution and/or an aqueous emulsion of a rubber solution is expediently employed as the rubber material. The rubber material may be a synthetic and/or naturally occurring rubber. According to a preferred embodiment of the invention, a styrene-butadiene copolymer is used as the rubber latex. Further, according to the invention, one or more fillers may be used. Carbon black, for example, may be used as the filler. In one embodiment of the invention, an aqueous suspension of carbon black is added to the rubber material, in particular a 2 to 7 wt. % aqueous suspension of carbon black is added to the rubber material.
The particle size range of the rubber powders obtained according to the invention is in general between 0.05 and 10 mm, in particular between 0.5 and 2 mm.
The pulverulent rubbers comprise 20 to 250 phr, in particular 50 to 100 phr of filler (phr: parts per hundred parts of rubber), and have optionally been modified partly or completely on the surface before the process according to the invention takes place, using the organosilicon compounds according to the formulae (I), (II) or (III), shown below, known in the rubber art. The following species, individually or as a mixture with one another, have been found to be suitable types of rubber for use in the invention: natural rubber, emulsion SBR with a styrene content of 10 to 50%, butyl-acrylonitrile rubber. Butyl rubbers, terpolymers of ethylene, propylene (EPM) and non-conjugated dienes (EPDM), butadiene rubbers, SBR, prepared by the solution polymerization process, with styrene contents of 10 to 25%, and contents of 1,2-vinyl constituents of 20 to 55% and isoprene rubbers, in particular 3,4-polyisoprene are useful in the invention.
In addition to the rubbers mentioned, the following elastomers may be used, individually or as a mixture: carboxyl rubbers, epoxide rubbers, trans-polypentenamer, halogenated butyl rubbers, rubbers of 2-chloro-butadiene, ethylene/vinyl acetate copolymers, epichlorohydrins, optionally also chemically modified natural rubber, such as, for example, epoxidized types. Fillers which are additionally used are in general the carbon blacks and white fillers of a synthetic nature, such as e. g. precipitated silicas, or naturally occurring fillers, such as e. g. siliceous chalk, clays etc., known from rubber processing.
Carbon blacks such as are generally employed in rubber processing are particularly suitable.
These include furnace blacks, gas and flame blacks with an iodine adsorption number of 5 to 1000 m2/g, a CTAB number of 15 to 600 M2/g, a DBP adsorption of 30 to 400 ml/100 g and a DBP number of 50 to 370 ml/100 g, in an amount of 5 to 250 parts, in particular 20 to 150 parts per 100 parts of rubber, in particular 40 to 100 parts.
The silicate fillers of synthetic or natural origin known from the rubber art, in particular precipitated silicas, are likewise suitable.
In general, these have an N2 surface area, determined by the known BET method, of 35 to 700 m2/g, a CTAB surface area of 30 to 500 m2/g, and a DBP number of 150 to 400 ml/100 g. The product according to the invention comprises these silicas in an amount of 5 to 250 parts, in particular 20 to 100 parts, based on 100 parts of rubber.
If the fillers are white, naturally occurring fillers, such as clays or siliceous chalks with an N2 surface area of 2 to 35 m2/g, these are employed in an amount of 5 to 350 parts, based on 100 parts of rubber.
Powders which comprise one or more of the abovementioned fillers in a mixture are also suitable.
In addition to the non-modified fillers of the type mentioned, modified fillers are optionally additionally employed in the preparation of the rubber powders described herein. The content of non-modified fillers depends on the mixture specifically to be prepared. In all cases, the total amount of filler amounts to 20 to 250 phr. This in general comprises the non-modified fillers: silica and/or carbon black to the extent of 100% in particular to the extent of 30 to 100%, preferably 60 to 100%. Organosilicon compounds which are employed for the modification of the surfaces are in general those of the general formulae
[R1nxe2x88x92(RO)3xe2x88x92nSixe2x80x94(Alk)mxe2x80x94(Ar)p]q[B1]xe2x80x83xe2x80x83(I),
R1n(RO)3xe2x88x92nSixe2x80x94(Alk)xe2x80x83xe2x80x83(II),
or
R1n(RO)3xe2x88x92nSixe2x80x94(alkenyl)xe2x80x83xe2x80x83(III),
in which the symbols denote
B1: xe2x80x94SCN, xe2x80x94SH, xe2x80x94Cl, xe2x80x94NH2 (if q=1) or xe2x80x94SXxe2x80x94(if q=2)
R and R1: an alkyl group having 1 to 4 carbon atoms, branched or unbranched, the phenyl radical, where all the radicals R and R1 in each case can have the same or a different meaning, preferably an alkyl group,
R: a C1-C4-alkyl, -C1-C4-alkoxy group, branched or unbranched,
n: 0; 1 or 2,
Alk: a divalent straight-chain or branched carbon radical having 1 to 6 carbon atoms,
m: 0 or 1
Ar: an arylene radical having 6 to 12 C atoms
p: 0 or 1, with the proviso that p and m do not simultaneously denote 0,
x: a number from 2 to 8,
alkyl: a monovalent straight-chain or branched unsaturated hydrocarbon radical having 1 to 20 carbon atoms, preferably 2 to 8 carbon atoms,
alkenyl: a monovalent straight-chain or branched unsaturated hydrocarbon radical having 2 to 20 carbon atoms, preferably 2 to 8 carbon atoms.
Modified fillers which are employed according to the invention are described e.g. in EP-B 0442 143, EP-B 0177 674, and in particular in the form of granules in EP-A 0795 579 (white fillers), or in EP-B 0519 188 (carbon black).
The bis(alkoxysilylalkyl)-oligosulfanes of the types bis(trialkoxysilylpropyl)-tetrasulfane and -disulfane have proved to be particularly suitable for the pre-modification or the addition to the filler suspension.
The modified fillers known from the patent applications or patents mentioned above and the organosilicon compounds mentioned there are expressly also included in the present application as a constituent of the compositions of the inventions.
In addition to the fillers already mentioned, the rubber powders according to the invention preferably comprise known processing or vulcanization auxiliary substances, such as zinc oxide, zinc stearate, stearic acid, polyalcohols, polyamines, resins, waxes, plasticizer oils, anti-aging agents against heat, light or oxygen and ozone, reinforcing resins, flameproofing agents, such as e. g. Al(OH)3 and Mg(OH)2, pigments, various crosslinking chemicals and accelerators and optionally sulfur in the concentrations conventional in rubber technology, preferably sulfur in modifications to which surface-active substances have been added such as are commercially available.
The particle size is determined from the filler suspension.
In a particularly preferred embodiment of the process according to the invention, all the solids employed are present in a particle size of  less than 50 xcexcm, preferably  less than 10 xcexcm, before precipitation of the rubber particles out of the suspension. The formation of agglomerates may optionally occur during the preparation, but these agglomerates do not adversely influence the processing properties.
The invention also provides a process for the preparation of finely divided, filler-containing rubber powders by precipitation, in a tube provided with throttle flaps at a suitable position, from aqueous mixtures which contain finely divided filler(s) optionally modified with organosilicon compounds, (carbon black and/or silicate filler) water-soluble salts of a metal of Groups IIa, IIb, IIIa and VIII of the Periodic Table of Elements and a rubber latex or an aqueous emulsion of a rubber solution, optionally in the presence of an organic solvent, which is characterized in that
a) xe2x89xa750 wt. %, but less than 100 wt. %, of the envisaged amount of the finely divided filler, preferably in the form of an aqueous suspension with a content of 2 to 15 wt. % in water, optionally with one of the amount envisaged for the modification of the filler surface of one or more of the organosilicon compounds according to the formulae (I), (II) or (III) in an amount of 0.1 to 20 wt. %, based on the filler, in particular if it is a silicate filler, preferably precipitated silica, and/or xe2x89xa750 wt. %, but less than 100 wt. %, of a filler which is at least partly modified on the surface with one or more of the organosilicon compounds (formulae (I), (II) or III), are mixed, preferably in the presence of an emulsifier, with a rubber latex or an aqueous emulsion of a rubber solution and the pH of the mixture is reduced to a value in the range of 7.5 to 6.5, preferably by addition of a Lewis acid, (first stage),
b) the remaining content of the abovementioned finely divided fillers, optionally together with the residual amount envisaged for modification of the filler surface of organosilicon compounds of the formulae (I), (II) or (III), is added in the form of a suspension, the pH is reduced, preferably by addition of a Lewis acid, to a value in the range of  less than 6.5 to xcx9c5, preferably xcx9c5.5, so that the rubber in the mixture precipitates completely together with the filler (second stage),
c) the solid which has precipitated is separated off by measures known per se,
d) and is optionally washed and
e) dried.
The organosilicon compounds are employed in particular if silicate fillers, preferably silicas, are used.
The precipitation process is in general carried out at room temperature, in particular at 20xc2x0 to 80xc2x0 C., in the tube, which is preferably provided with several input positions.
The process is carried out by adaptation of the methods known from DE 198 58 706.6.
In the process of the invention, the tube is preferably a tube of circular cross-section. The tube diameter is, for example, 10 to 30 mm. The total tube length is preferably 1 to 3 m, more preferably 1.5 to 2.5 m.
According to the invention, the tube cross-section is reduced at least at one throttle position. Preferably, the diameter of the tube before the throttle position is the same as the diameter of the tube after the throttle position. The invention also optionally provides an adjustable throttle control located at the throttle position. The throttle control is preferably a valve, more preferably a squeeze valve with an elastic insert. According to a preferred embodiment of the invention, the throttle control, of which there is at least one, is set or adjusted with the aid of a drive device. The purpose of the variable reduction in the cross-section at the throttle position is to allow the continuous process always to be conducted such that the drop in pressure achieved at the throttle position is greater than 0.5 bar. In other words, the throttle cross-section is expediently always adjusted with the adjustable throttle control during the continuous process procedure such that the pressure before the throttle position or before the throttle control is at least 0.5 bar higher than after the throttle position or after the throttle control. The drop in pressure in the liquid mixture achieved with the throttle position is not more than 6 bar. Preferably, a drop in pressure of 0.5 to 3 bar, more preferably 0.8 to 2.5 bar, is achieved at the throttle position. Because of the throttling, a surprisingly optimum mixing of the components passed through the throttle position is achieved. It lies within the context of the invention that the tube cross-section widens again in the direction of flow after a throttle position according to the invention.
According to another preferred embodiment, the throttle cross-section is varied during the continuous process procedure in order to remove deposits in the throttle region. Deposits in the throttle region mean, in particular, deposits or caking on the walls shortly before the throttle position and shortly after the throttle position and in the throttle control. Thus, if deposits or caking are found in the throttle region, the throttle cross-section is preferably varied as a function thereof. Regulation of the throttle cross-section is expediently automatic.
According to a further preferred embodiment, a constant drop in pressure is always maintained or set at the throttle position in the context of the process according to the invention. Thus, if the drop in pressure at the throttle position changes because of deposits or caking, the throttle cross-section is preferably readjusted until the desired constant drop in pressure is achieved again. A surprisingly effective mixing can thus be achieved for the components employed for the preparation of the finely divided rubber powder with the throttling according to the invention, so that end products with the desired properties can be prepared without problems. It is advantageous that deposits or caking which would disadvantageously change the flow circumstances can be prevented according to the invention in a simple manner and reproducible products can thus be obtained.
According to one embodiment of the invention, the liquid mixture of rubber latex and filler suspension is passed through a tube with a throttle position before addition of coagulant. In this embodiment, effective premixing of the rubber latex and filler takes place immediately after the throttle position. According to another embodiment of the invention, a liquid mixture of rubber latex, filler and coagulant is passed through one of the abovementioned throttle positions. It lies within the context of the invention to provide a first throttle position through which a liquid mixture of rubber material and filler suspension is passed, and thereafter to add coagulant, and only then to pass this liquid mixture through a second throttle position. Effective mixing of the rubber material, filler and coagulant takes place immediately after this second throttle position. According to a preferred embodiment of the invention, both the throttle cross-section of the first throttle position and the throttle cross-section of the second throttle position are adjusted such that the drop in pressure in the liquid mixture achieved with the particular throttle position is greater than 0.5 bar and not more than 6 bar.
According to a preferred embodiment, which is of particular importance in the context of the invention, the precipitated rubber particles are coated with a coating composition. The coating composition is added to the coagulated rubber material. A filler for rubber is expediently employed as the coating composition. The filler which was already mixed with the rubber material at the start of the process can be employed here. However, another filler may be used.
According to another preferred embodiment of the invention, the liquid mixture with the added coating composition is passed through an additional throttle position in the tube. The drop in pressure at this throttle position is also expediently more than 0.5 bar and not more than 6 bar, preferably 3 bar.
According to a further preferred embodiment of the invention, the liquid mixture is passed through a total of three throttle positions in the tube. In this embodiment, the rubber material and filler are first mixed and passed through a first throttle position. In the subsequent section of the tube, the coagulant is added and the resulting liquid mixture is passed through a second throttle position. In the subsequent tube section, the precipitation of rubber particles takes place, and the coating composition is then added in this tube section. The mixture with the coating composition is then passed through a third throttle position. The drop in pressure at each of the three throttle positions is preferably more than 0.5 bar. At least two, preferably all three, throttle positions are expediently constructed as has been explained above for a first throttle position.
According to a second preferred embodiment, two throttle positions in total are provided in the tube. In this embodiment, the rubber material, filler and coagulant are first mixed and this liquid mixture is passed through the first throttle position. In the tube section after the first throttle position, the precipitation of the rubber particles then takes place, and the coating composition is subsequently added. The mixture with the coating composition is then passed through a second throttle position. The throttle cross-section at each of the two throttle positions is preferably adjusted with the proviso that the drop in pressure in the liquid mixture achieved with the particular throttle position is greater than 0.5 bar and not more than 6 bar. The two throttle positions are expediently constructed as has been explained above for a first throttle position.
The invention also provides a device for carrying out the process according to the invention.
The invention is based on the finding that a filler-containing rubber powder with reproducible properties can be obtained in a manner which is not particularly expensive if the continuous procedure according to the invention is used. A surprisingly effective mixing of the components employed here (rubber material, filler, optionally coagulant) is achieved using the throttle positions according to the invention, so that thereafter the desired reaction proceeds completely and without problems. In the case of the components used here, in particular, trouble-free mixing is generally problematic, for example, in achieving a completely homogeneous embedding of the filler in the rubber. With the variable throttling according to the invention, this mixing is achieved in a manner which, to the expert, is surprisingly effective and functionally reliable. A specific turbulent flow is generated with the aid of the throttle controls, preferably throttle valves, because of the drop in pressure according to the invention and, as a result, a very homogeneous mixture is achieved, so that homogeneous end products also result. This was not possible with the known processes described above. In addition to optimum mixing of the components, as a result of the throttling according to the invention, deposits or caking in the region of the throttle position can furthermore be avoided surprisingly effectively by changing the throttle cross-section. By regulating the pressure loss with the aid of variable throttle cross-sections, throttle positions which are insensitive to blocking and function as optimum mixing controls are thus obtained according to the invention. Pulverulent rubbers which can already comprise all the rubber auxiliary substances can be prepared by the process according to the invention. In addition to fillers, these auxiliary substances are, for example, sulfur, vulcanization accelerators, activators and rubber anti-aging agents. These rubber auxiliary substances can be added to the liquid mixture without problems during the continuous process procedure, and these rubber auxiliary substances are then present in the pulverulent end products in a very homogeneous distribution. In contrast to known processes, after-treatments, in particular coating of the rubber particles, are also possible in a simple manner in that a further throttle control is provided according to the invention and the drop in pressure established at this throttle control contributes towards effective mixing also of the coating composition with the liquid mixture and thus functionally reliable and uniform coating of the rubber particles. With the continuous procedure in the tube according to the invention, considerably higher yields per unit time are achieved compared with a discontinuous procedure. The residence times of the liquid mixture in the tube according to the invention are relatively short.